@Alexandre. 

Thank you for the suggestion. I think this would indeed work (even though my dad says the main router is the problem and sometimes messes up the IP assignment of the devices)

I have another question that comes after I read this.

https://www.tp-link.com/us/support/faq/1794/

Mainly I think my main router is a bit crappy, it's from the internet provider and they really configure it in a way that I can't change most things and I don't understand if I can do what I want with the mesh setup

I see in my main mesh that I connect to the router I have 2 Ethernet connections. Can I connect a switch to the second Ethernet and then manage all other connections from that switch?

My idea would be to romeve everything from the main router, use that only for providing the internet connection, and then everything else be in a network managed by my deco device. However I don't understand the point about Ethernet backhaul and what that actually means. Will the Deco stop providing a wifi network if it's providing a Ethernet connection also?

I guess my question is as simple as from the 2 Ethernet connections can I use one as input and two as output?

On my power adapters that works but normally they say which is input and which is output

I think I want to do something like this

https://community.tp-link.com/en/home/forum/topic/207020

The main difference is I want to change the main switch by my main Deco and have everything else in other switchs that are all managed by the main Deco

Not sure this is smart idea or not.

@cstmgl 

If you want to configure your home network like this, it should work:

Still, you may be adding unnecessary complexity by running Deco in Router mode, behind ISP Router. Also, you are not removing ISP Router from this configuration, so why not to use it if it is there anyway?

When I got my Deco mesh, I set it in Access Point mode and disabled WiFi on my ISP Router. I have found ISP Router performance improved when it does not have to take care of WiFi, also ISP Router used to take up to 5 minutes for reboot and now it reboots under 1 minute.

My ISP router has 4 Ethernet ports, I am using three of them. With Deco mesh in AP mode it is seamless. 

Anyway, your home network - your choice. 

>On my power adapters that works but normally they say which is input and which is output

Deco will autosense which of its two Ethernet ports are connected to ISP Router and which - to home network. You do not need to worry about it.